If you mean you when yous say 3000 tr/min you are using 3000 Revolutions
Per minute (RPM) in a reasonably large centrifuge the G force is very
high. If your cells are just in saline they may be being crushed to
death. 12,000 RPM for 3 minutes in a centrifuge with 12 cm radius is
easily sufficient. Also saline is not the best medium for preserving
cell viability-use a recognized medium for lymphocytes such as RPMI 1640
with 10% Fetal calf serum.